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08-07-2009, 08:55 PM   #1
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Pentax-A 28-80 3.5-4.5 but f3.5 not recognized?

Hello wizards,
I recently acquired a SMC-A 28-80mm zoom that although the aperture ring clicks down to 3.5, my K20D can only recognize aperture as wide as 4.0 regardless of focal length when the ring is set to its "A" setting. I am told this is common with Pentax-A zooms. Can anybody confirm or reject the fact that this zoom is not defective and that it is only usable to f4.0 with any camera? Otherwise the lens seems fine and runs up to f22 and down to f4, just not to f3.5 as spec'ed and as on the the ring.

What am I missing?

edit:
I can return the lens if I need to but I am trying to determine if its defective or if thats just the way it works....ie, you only get f/4 even though its a 3.5-4.5 lens. Looking for someone who actually has the lens and if a K20D then perfect situation. Is this just the way an "A" zoom lens works? the variable aperature becomes a constant?


Last edited by imtheguy; 08-07-2009 at 10:42 PM.
08-08-2009, 09:10 AM   #2
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It ought to work. The K20D can certainly work with a variable aperture zoom.

The problem is likely an electrical contact not working. The contacts tell the camera what the minimum and maximum aperture is. If one is dirty or broken or reassembled incorrectly, the camera won't read it right.

Set the aperture ring to A, lens to 28mm, set the aperture on the camera as low as it can go, then use the DOF preview. Look into the front of the lens and see if you see aperture blades. If your camera can only recognize f3.5 as f4.0, there should be no difference between engaging DOF preview and not.

You can try setting the aperture ring to 3.5, lens to 28mm, camera to M mode, and use the DOF preview. The aperture blades shouldn't be visible here either. That method uses only the aperture mechanism, no electrical information.
08-08-2009, 08:11 PM   #3
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I use to wiggle the lens slightly back and forth on the mount with previous cameras, although not with the K20d, and I could get the value to change on the camera. I haven't really tested the K20d's mount, but with the weathersealing everywhere, there may not be as much play as with older K-mount bodies that had ever so slight play after mounting. It's worth a try, as maybe the older lens isn't quite lining up perfectly with the K20d's mount.
08-08-2009, 10:05 PM   #4
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QuoteOriginally posted by Just1MoreDave Quote
Set the aperture ring to A, lens to 28mm, set the aperture on the camera as low as it can go, then use the DOF preview. Look into the front of the lens and see if you see aperture blades. If your camera can only recognize f3.5 as f4.0, there should be no difference between engaging DOF preview and not.
Thanks for something to try. I could hear the aperture engage when I hit the DOF lever but did not see any movement.

QuoteOriginally posted by K-9 Quote
I use to wiggle the lens slightly back and forth on the mount with previous cameras, although not with the K20d, and I could get the value to change on the camera. I haven't really tested the K20d's mount, but with the weathersealing everywhere, there may not be as much play as with older K-mount bodies that had ever so slight play after mounting. It's worth a try, as maybe the older lens isn't quite lining up perfectly with the K20d's mount.
Thanks for the tip. It certainly gets the Aperture info ok for f4-f22 but wiggling and cleaning mount did not change anything. Appreciate the help.

08-09-2009, 10:50 AM   #5
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Case closed for now as I figured out what I wanted to know:

I compared the metering of a few other lenses at 28mm and f4 to the zoom in question and it appears the zoom really is working at f3.5 even though the exif shows f4. I used a DA 16-45mm and a 28-135mm both set at 28mm and f4 (camera Ap mode).
Not exactly a perfect lab situation but the light was pretty dim and controlled. Using all three lenses I then metered for the same shot and the zoom in question was 1/15 sec shutter while the other two lenses were 1/8-1/10 over mutltiple tries.
Is that enough to mean the 28-80 is really working at f3.5 but just can't communicate it? Seems that way to a slow learner like me.
08-10-2009, 06:38 PM   #6
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QuoteOriginally posted by imtheguy Quote
Is that enough to mean the 28-80 is really working at f3.5 but just can't communicate it? Seems that way to a slow learner like me.
I think so.

I finally realized that I have a variable-aperture KA-mount zoom to test with my camera. I am wrong about the camera knowing it's variable aperture. It only seems to know the widest possible aperture. Later autofocus lenses seem to be able to communicate the aperture changes to the camera along with focal length.
08-10-2009, 07:36 PM   #7
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QuoteOriginally posted by imtheguy Quote
Is that enough to mean the 28-80 is really working at f3.5 but just can't communicate it?
Yes, that's right. I had the same lens and the same 'problem' some years ago and I discovered that A-lenses do not pass on exact aperture info (with variable aperture lenses) to the camera's viewfinder display due to the technology limitation of the day. This doesn't affect exposure however as you've discovered.
08-10-2009, 08:27 PM   #8
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All A lenses can only report one set of max/min apertures. So your lens is perfectly functional.

At 28mm, if you set it to F4, it is shooting at 3.5, and if you set it to 4.5, then it's actually F4, etc. At 80mm, F4 = F4.5, F5.0=F5.6, etc.

The number you see in the viewfinder is only a label. What is important is how far you've told it to stop down, so that is why autoexposure works. All F5.6 means in your lens' case is that the camera will stop the lens down the amount from F4, which is one stop, regardless of what wide open is for your lens.

This will be a small problem if you use any metering not built into the camera, such as a light meter, or flash GN table, but the difference is only 1/3 stop either way.

08-10-2009, 09:17 PM   #9
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